Method of relieving residual stresses in light metal articles



METHOD OF RELIEVING RESIDUAL STRESSES 1N LIGHT METAL ARTICLES Lowell A. Willey, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Dec. 18, 1958, Ser. No. 781,198

Claims. (Cl. 148-125) This invention relates to a method of relieving residual stresses in light metal articles and it is more particularly concerned with substantially reducing residual stresses in aluminum or magnesium base alloy products which have been quenched from a solution heat treating tem perature. The term light metal, as used herein, refers to aluminum and magnesium and the alloy wherein these metals constitute a major portion of the composition.

When light metal articles are quickly cooled from a high to a low temperature stresses are developed within the article by virtue of the difierence in cooling rate across the cross section. Initially, the hot center of the article undergoes plastic compression but after the center ceases to be plastic as a result of cooling, stresses are created which are compressive at the surface and tensile at the center. The presence of such stresses is sometimes regarded as being undesirable, especially if warpage results from redistribution of these stresses when the articles are machined. Various means have been proposed for reducing these stresses such as a less drastic quenching or a mechanical treatment after quenching. The employment of such quenching media as boiling water or hot oil pressing is somewhat limited in its application by the shape and size of the article.

Residual stresses may also be developed in light metal articles by cold working as by rolling, drawing, sinking.

and the like. The magnitude of the internal stresses depends upon the reduction, usually a light reduction develops greater residual stresses than a heavy reduction. The residual stresses are not to be confused with the stresses that produce cold working strains with a resultant increase in the hardness and strength of the light metal article. Broadly speaking any stresses that cause warpage of an article when a portion thereof has been removed, such as occurs during machining, are re garded as constituting residual stresses.

It is an object of this invention to provide a method of treating light metal articles having residual stresses therein which will substantially reduce those stresses.

Another object is to provide a method of relieving residual stresses in light metal articles which have undergone a conventional solution heat treatment and subsequent quenching.

Still another object is to provide a method of relieving residual stresses in conventionally quenched light metal articles which does not involve mechanical treatment of the articles.

These and other objects and advantages are attained by chilling the light metal articles containing residual stresses to a temperature below 100 F. followed by a very rapid reheating to at least room temperature as achieved by the condensation of a vapor, such as steam,

upon the surface of the articles. It has been found that 2,949,392 Fatented Aug. 16, 1960 by quickly heating the articles from a temperature below l00 F. in a current of steam a very substantial reduction in residual stress is efiected. Such treatment apparently neutralizes existing stresses by super imposing what is known as a negative stress pattern. There is some evidence that the stresses at the surface of the article are changed from compressive to tensile while those at the center are converted from tensile to compressive. a

In applying the quick reheating treatment to light metal articles which have been solution heat treated and quenched, it is necessary that the chilling to sub-zero temperatures be done within a short time after quenching before any substantial precipitation occurs. It has been found that if precipitation takes place to any substantial extent, the benefit from the sub-zero chilling and minute.

quick reheating is greatly reduced. In order for the stress relieving treatment to be most efiective the heat treated alloy must be in a relatively soft condition (is. possess a low yield strength) as compared to the hardness attainable by subsequent precipitation treatment. To secure the best results by the present process the quenched articles should be chilled to-a sub-zero temperature before the inception of any substantial amount of age hardening. In the case of spontaneously age hardening alloys the quenched article should be refrigerated or precipitation otherwise arrested. In the absence of refrigeration or other means of arresting precipitation it is advisable to chill the article within one hour after removal from the quenching medium. In treating articles having residual stresses produced by other means than by quenching, for example by cold working, the stress relief treatment may be carried out at any convenient time after the operation which has created the residual stresses.

The articles having residual stresses therein must be chilled to a temperature below l00 F. in order to gain the benefit from the stress relieval treatment. It is generally advisable to chill the articles to even lower temperatures and thereby increase the range over which the article will be reheated. The rate of reheating from the low temperature is very important and it must be sutliciently rapid to produce a thermal gradient which causes some plastic deformation in the metal, that is, the stresses produced by the reheating must be great enough to exceed the elastic limit of the metal. The temperature gradient established between the surface and central portion of the article should be at least F. It has been found that particularly good results are obtained where the articles are immersed in liquid nitrogen and brought to a temperature of 320 F. It is possible that under some conditions additional advantages would be obtained by cooling to still lower temperatures, such as to 452 F. with liquid helitun. Various chilling media may be used depending upon the temperature to which the article is to be cooled. Dry Ice, or a mixture of Dry Ice and acetone, ether or trichlorethylene, will reduce the temperature to -l08 F. or lower. Liquid isopentane will chill to a temperature of 256 F. In carrying the chilling to lower temperatures, it is usually advisable to conserve the liquid gas by first cooling the articles to an intermediate temperature of -l00 to 200 F. and then subjecting them to further chilling in a lower boiling point medium such as chilled iso-pentane, liquid air, liquid nitrogen, or liquid helium.

After the articles have reached the desired low temperature, usually the temperature of the chilling medium, they are removed therefrom and immediately exposed to a rapidly moving stream of condensable vapor, such as steam, at a temperature not lower than the boiling point of water, preferably for a period of less thankl- The vapor should be kept in motion across the entire surface of the article. The rapid condensation of the vapor and the continuous supply of additional vapor serves to raise the surface temperature of the articles very rapidly thereby creating a sharp temperature gradient between the surface and central portion of the article. Exposure to a relatively quiescent atmosphere does not produce the desired results, there must be a current or stream of the heating medium directed over the chilled articles with sufficient velocity to sweep away any droplets of liquid condensate. The vapor employed should be chemically stable and substantially inert toward the alurninous metal. Furthermore the vapor should have a relatively high heat of condensation to supply a large amount of heat to the articles being heated. Such vapors as steam, alcohol, glycerine, glycols, and their alkyl substituted products may be employed, but steam is preferred;

The vapor, and particularly steam, should be at a temperature above the boiling point of water and preferably above 240 F. The higher steam temperatures available in superheated steam are advantageous in promoting a more rapid reheating of the articles. To obtain a rapidly moving stream over substantially the entire surface of the articles, the vapor must be projected over it at super atmospheric pressure, preferably at more than p.s.i. above atmospheric pressure since the articles are initially surrounded by air at atmospheric pressure. The pressure, in any event, should be sulficient to blow off a substantial portion of the liquid condensate. The pressures will, of course, be greater the higher the temperature of the steam employed. A very effective means of obtaining a rapid flow of vapor is to direct jets toward the articles being heated. While the vapor pressure need not drop to atmospheric pressure to obtain the desired heating, yet for practical reasons it is advisable to allow the pressure adjacent the articles to reach approximately the atmospheric value. If the treatment is conducted in a chamber, the flow of vapor can be easily controlled.

It will be appreciated that while benefits may be derived from directing a flow of vapor at a previously chilled article in the open air, better and more uniform results are obtained if this is done within a chamber.

The chilled articles should be heated by the vapor to a temperature not lower than room temperature but it is generally more desirable that it be heated to a temperature as high as that of the vapor. Generally, it is not advisable to maintain the vapor heated articles at an elevated temperature for a sufliciently long time to produce any substantial precipitation hardening in that type of alloy. It is more practical and efiicient to remove the articles from the vapor atmosphere and perform the precipitation hardening treatment in a furnace. Precipitation hardening may be of the conventional type, for example the articles may be heated for a period of from 1 to 24 hours at 250 to 400 F. depending upon the alloy and size of the article being treated, or they may age spontaneously. In either case the hardening does not affect the relief of stresses obtained by the preceding treatment.

The stress relieving treatment described above is particularly applicable to those light metal articles which may have been cooled from an elevated temperature at a sufliciently rapid rate to create residual stresses. The most common treatment where this is practiced is that of quenching articles from a solution heat treating tem perature to develop a high strength and hardness. The aluminum base alloys generally subjected to such a treatment are of the aluminum-copper, aluminum-magnesium-silicon, and aluminum-zinc-magnesium types. In the aluminum-copper type the copper content may be between 2.5 and 10% and in the aluminum-magnesiumsilicon type magnesium is generally within the range of 0.4 to 1.5% and the silicon content is 0.2 to 1.2%. In the aluminum-zinc-magn'esium type 3 to 8% zinc and 1 to 3% magnesium are employed. It is well-known that the foregoing types can be modified by the addition of other elements such as manganese, chromium, titanium, boron, etc. The magnesium base alloys which are frequently subjected to heat treatment are of the following types: magnesium-5 to 12% aluminurnup to 0.5% manganese; the same base plus 0.5 to 5% zinc; magnesium2 to 8% zincup to 1% zirconium; the same base plus 1 to 5% of the rare earths; and magnesium-1 to 5% rare earthup tov 1% zirconium.

Non-precipitation hardenable alloys that may be employed are of the types aluminum-1 to 2% manganese and aluminum-1 to 5% magnesium.

The articles to be treated may be in the form of wrought or cast products.

The benefit to be derived from the invention is illustrated in the following examples.

The specimens used consisted of blocks cut from hot rolled plate, the blocks being 12" x 6" x 2.. After treatment a 1 wide strip down the center of each specimen was used as the sample from which the evaluation of the residual stresses was made. The material on either side of the strip was used for tensile property determinations. The residual stresses were measured by a Whittemore strain gage between points 10" apart on the upper and lower surfaces of the 1 wide strip sawed therefrom after initial readings had been made. Any change in curvature and length of the sawn strip was noted. In the next step, A" thick strips were sawed from the upper and lower surfaces of the 1 wide strip, also a A thick strip was taken from the middle of the test piece and changes in length measured. In this manner the distribution of residual stresses was determined.

Two aluminum base alloys were employed in the tests, one (A) had a nominal composition of 4.4% copper, 0.8% silicon, 0.8% manganese, 0.4% magnesium and balance aluminum, and the second alloy (B) had a nominal composition of 5.6% zinc, 2.5% magnesium, 1.6% copper, 0.3% chromium and the balance aluminum. The blocks of alloy A were solution heat treated 3 hours at 940 F. and quenched in cold water while the blocks of alloy B were heat treated 3 hours at 870 F. and quenched in cold water. One block of each alloy was immediately precipitation hardened, that of alloy A being heated for 10 hours at 340 F. soon after quenching and that of alloy B was heated 36 hours at 250 F. 4 days after quenching. The remaining blocks were cooled to sub-zero temperatures within a period of /2 to 1 /2 hours after the quench in cold water.

The chilling was done by immersing the blocks in a Dry Ice-trichlorethy-lene mixture and holding them therein for 10 minutes so that they reached a temperature of 108 F. Some blocks were further cooled by transferring them to liquid nitrogen and allowing them to remain in the liquid for 5 minutes until they attained a temperature of 320 F.

A portion of the chilled blocks was removed from the cooling medium and transferred to an open chamber where they were immediately reheated by a blast of steam from jets. The steam when released was under a pressure of'about 1S0 p.s.i. above atmospheric pressure. The blocks were subjected to the steam blast for 30 seconds which raised their temperature to 210 F. Other chilled blocks were immersed in boiling water -for a period of 3 to 4 minutes. The blocks were then given the precipitation treatment at 340 and 250 F. referred to above. The residual stresses and tensile properties of the blocks were then determined. The results of the residual stress tests. are given in Table I below, the blank specimens being those which had been precipitation hardened, without chilling and rapid reheating. The residual stress range is the span between the maximum compressive value near the surface and the maximum tensile value at the center of the specimen.

TABLE I Residual stresses in quenched and treated specimens Residual Reduction Test Alloy Cooling Heating Temp. Stress in Residual N0. Medium Medium Range, F. Range, Stress,

p.s.i. Percent 1 A 24, 000 2 A Dry Ice Boiling H2O l to 210 19, 000 19 3 A Liquid N2... Boiling H20 320 to 210 19,000 19 4 A Dry Ice Steam 100 to 210 12, 400 48 5 A Liquid Ni... Steam -320 to 210 4, 000 83 6 B 000 7 B Dry Ice. Steam 100 to 210 15, 800 50 8 B Liquid N Steam 320 to 210 5, 600 84 It will be observed that the boiling water .did not have much of an effect upon relief of residual stresses while the steam blast was much more effective. It is also significant that reheating with steam from a temperature of 320 -F. was most effective in reducing residual stresses. This indicates that a rapid reheating over a wide temperature range is most beneficial.

The mechanical properties of specimens taken from the same block as those referred to above are given in Table II below.

From the standpoint of mechanical properties it will be seen that the cooling to very low temperature and rapid reheating produced but an insignificant change in strength. The low elongation value in test 5 is not considered to reflect any marked change in ductility.

Having thus described my invention and certain embodiments thereof, I claim:

1. The method of relieving residual stresses in a light metal article comprising chilling said article to a temperature below 100 F. removing the article from the chilling medium and immediately after removal therefrom directing a current of condensable vapor having a temperature not lower than the boiling point of water over substantially the entire external surface of the article and at a velocity which sweeps away droplets of liquid condensate whereby the surface of said article is heated at a sufficiently rapid rate as to produce plastic flow in the metal.

2. The method of claim 1 wherein the condensable vapor is steam at a temperature not lower than 240 F.

3. The method of claim 1 wherein the article is exposed to the condensable vapor for a period of not longer than 1 minute after removal from the chilling medium.

4. The method of relieving residual stresses in a light metal article comprising immersing said article in a mixture of Dry Ice and trichlorethylene whereby the temperature of the article is reduced to below 100 F., removing said article from the chilling mixture and immediately after removal therefrom projecting a current of steam at a temperature of not less than 240 F. over substantially the entire external surface of the article and at a velocity sufficient to sweep away any droplets of water whereby the surface of said article is heated at a. sufiiciently rapid rate as to produce plastic flow in the metal.

5. The method of relieving residual stresses in a light metal article comprising immersing said article in a liquefied gas zbath whereby the temperature of said article is brought to a temperature below -10() F, removing the article from the bath and immediately after removal projecting a current of steam at a temperature of not less than 240 F. over substantially the entire external surface of the article and at a velocity sufficient to sweep away any droplets of water whereby the surface of said article is heated at a sufiiciently rapid rate as to produce plastic flow in the metal.

6. The method according to claim 5 wherein the liquefied gas is nitrogen.

7. The method of relieving stresses in an article composed of a precipitation hardenable light metal alloy which has been quenched from the solution heat treating temperature comprising chilling said :article to a temperature below l0O F. before the inception of any substantial precipitation hardening, removing said article from the chilling medium and immediately after re moval therefrom projecting a current of condensable vapor having a temperature not lower than the boiling point of water over substantially the entire external surface of the article and at a velocity which sweeps away any droplets of liquid condensate whereby the surface of the article is heated at a sufliciently rapid rate as to produce plastic tflow in the metal.

8. The method of claim 7 wherein the precipitation hardenable alloy is an aluminum base alloy.

9. The method of relieving residual stresses in a light metal article comprising initially chilling said article to a temperature between and -200 F. and further chilling the article to a lower temperature by immersing in a bath of liquefied gas, removing said article from said bath and immediately after removal therefrom projecting a current of a condensable vapor over substantially the entire external surface of the article and at a velocity sufficient to sweep away any droplets of liquid condensate whereby the surface of said article is heated at a suificiently rapid rate as to produce plastic flow in the metal.

10. The method of claim '1 wherein the vapor is projected at the chilled article at a pressure of more than 10 psi. above atmospheric pressure.

References Cited in the file of this patent Tietz et al: Journal of Metals, vol. 1, No. 12, December 1949, pages 921-926.

Iron Age, June 17, 1943, pages 66 and 67. 

1. THE METHOD OF RELIEVING RESIDUAL STRESSES IN A LIGHT METAL ARTICLE COMPRISING CHILLING SAID ARTICLE TO A TEMPERATURE BELOW -100*F. REMOVING THE ARTICLE FROM THE CHILLING MEDIUM AND IMMEDIATELY AFTER REMOVAL THEREFROM DIRECTING A CURRENT OF CONDENSABLE VAPOR HAVING A TEMPERATURE NOT LOWER THAN THE BOILING POINT OF WATER OVER SUBSTANTIALLY THE ENTIRE EXTERNAL SURFACE OF THE ARTICLE AND AT A VELOCITY WHICH SWEEPS AWAY DROPLETS OF LIQUID CONDESATE WHEREBY THE SURFACE OF SAID ARTICLE IS HEATED AT A SUFFICIENTLY RAPID RATE AS TO PRODUCE PLASTIC FLOW IN THE METAL. 